Microwave probe for measurement of dielectric constants

ABSTRACT

A microwave probe for measuring the amount of soot or water in engine crankcase oil and/or detecting the level of oil comprises a coaxial cable having a tip including five substantially parallel wires shorted together at one end and connected at the other end to the coaxial cable, one of the wires being connected to the center wire of the coaxial cable and the other four wires being connected to the sheath of the coaxial cable. The size and geometry of the wires is selected so that the impedance of the tip when immersed in oil substantially matches the impedance of the coaxial cable.

This invention relates to a microwave probe for measurement ofdielectric constants and, more particularly, to such a probe for themeasurement of contaminants in engine crankcase oil and for thedetection of oil level.

It has been found that microwave sensors responsive to the dielectricconstants of a fluid are useful in a number of ways to inspect thecondition of the crankcase oil of an internal combustion engine. Forexample, when an engine is newly built and first tested a gasket orcasting leak can result in water entering the crankcase oil. Thus as aninspection method it is desirable to insert a probe into the crankcasethrough the dipstick access port to measure the dielectric constant ofthe oil for the detection of 1/2% of water in the oil. As anotherexample, in the case of diesel engines, soot build-up in the oil shouldbe detected to determine when to change the oil. A method of detectingsoot in engine oil using microwaves is set out in the U.S. Pat. No.4,345,202 issued to Nagy et al. Still another application of sensing thedielectric constant of the fluid in an engine crankcase is themeasurement of the oil level wherein the presence of air or vapor in theprobe along with oil provides a marked difference of dielectric constantfrom that of a probe fully immersed in oil. While there have been othermicrowave sensors for measuring the dielectric constant of liquids, inparticular, or of fluids, there are some particular constraints whichmust be observed for the successful application of such measurements inan engine crankcase. One is that since oil in some circumstances israther viscous there is a tendency for the oil to hang up on or in ameasurement probe so that as oil is changed or the contamination of oilchanges some of the old oil tends to remain in the probe to give rise tosome error in the measurement of the current oil condition. Thus theprobe must be constructed to allow very free oil flow-through and tominimize any pockets or other structure which would encourage hang up ofoil in the probe. Also the probe must be rugged and yet small enough tobe inserted through a dipstick access hole or passage tube for entryinto the crankcase, and the probe should be insensitive to metal objectsnear the probe location. In addition, for maximum sensitivity, thecharacteristic impedance of the probe tip must be matched to that of theassociated transmission line. Finally, where the probe is to be used asa permanent part of the vehicle equipment it should be simple andinexpensive to manufacture.

It is therefore a general object of this invention to provide amicrowave probe which facilitates free flow of liquid therethrough andhas an impedance which matches that of the associated transmission line.

A further object of the invention is to provide such a probe which issmall, rugged and of inexpensive construction.

It is a further object of the invention to provide such a probe which isnot affected by the presence of nearby metal objects.

The invention is carried out by a microwave probe having a coaxial cablewith an outer sheath and a central conductor and a tip attached to theend of the cable including a center wire joined to or comprising anextension of the center conductor and a plurality, preferably four,outer wires connected to the sheath and connected at their distal endsto the center conductor by a short, the wire size and geometry beingsuch that the impedance of the tip substantially matches thecharacteristic impedance of the coaxial cable.

The above and other advantages will be made more apparent from thefollowing specification taken in conjunction with the accompanyingdrawings wherein like reference numerals refer to like parts and wherein

FIG. 1 is a block diagram of a microwave circuit coupled to a microwaveprobe in an engine crankcase, and

FIG. 2 is an isometric view of a microwave probe according to theinvention.

FIG. 1 shows a partly broken away cross-sectional view of an internalcombustion engine housing 10 containing oil 12 in the crankcase and anopening 14 for a dipstick, the opening being optionally provided with atube 16 fitting within the opening to provide a passage for a dipstickas is well-known in the prior art. A microwave probe 18 comprising asemi-rigid coaxial cable 20 and a cage-like tip 22 on its lower endextends into the passage formed by the opening 14 and tube 16 so thatthe tip 22 is positioned in the crankcase preferably just at the minimumdesired level of the oil. A collar or stop 24 secured to the probe andpositioned to seat against the housing at the outer surface of theopening 14 establishes the position of the tip 22. The tip position inthe vertical sense is important where the probe is to be used as an oillevel sensor. If, however the probe is only to have a contaminationsensing function, then it may well be placed lower in the crankcasesomewhat below the minimum liquid level. A microwave oscillator 26 isconnected by means of waveguides 28 through an isolator 30 to the probe18. A line coupler 32 inserted in the circuit comprises an antenna wireextending into the waveguide to sense the value of the standing wave atthat point, the standing wave being a function of the microwave energyreflected from the probe. A diode detector 34 connected to the linecoupler 32 by a coaxial cable 35 provides a measure of the microwaveenergy reflected from the probe 18 and a meter 36 connected theretodisplays an indication of the condition of the oil 12 in terms ofcontaminant content and/or the liquid level.

The detailed structure of the probe is shown in FIG. 2. The coaxialcable, preferably a conventional 50 ohm cable, is illustrated as acylindrical conductive sheath 40 and a central conductor 42 and thespace between the two is filled with a dielectric material 44 all inaccordance with conventional coaxial cable technology. The tip 22comprises five substantially parallel wires including a center wire 46and four outer wires 48. The center wire 46 may be a continuation of thecenter conductor 42 or it may be a separate wire welded to the conductor42. The outer wires 48 are each joined as by welding to the outer sheath40. A conductive shorting disc 50 is welded at its center to the centerwire 46 and at its rim to the distal end of each outer wire 48. Thus theprobe is a cage-like structure characterized by a plurality ofwidely-spaced wires shorted at one end in an open configuration to allowfree oil flow through the tip without pockets to trap oil and alsowithout screens or mesh walls which might clog with foreign particles orotherwise impede the free flow of oil through the tip. The shorting disc50 could be replaced by another configuration of conductor such as asphere.

It is important for the tip to have an impedance which substantiallymatches that of the coaxial cable 20 when the tip is immersed in oil. Ifthe tip and cable have impedances within about 10% of each other, theyare substantially matched. Matching the tips characteristic impedance tothat of the coaxial cable increases the dependence of the resultingvoltage standing wave within the cable on the dielectric medium withinthe tip to increase detection capability. If the match is perfect thesensitivity is maximized, but some tolerance is allowable in a practicalsystem. In addition, when the tip is filled with oil and matched withthe cable impedance the only significant discontinuity in the probe isat the short, whereas if the tip had a substantially different impedancefrom the cable the interface between the cable and the tip would providea second discontinuity. Two discontinuities would result in a compositereflecting signal making the detection and analysis of the signaldifficult whereas the matched impedance tip offers no such difficulty.From transmission line theory it is known that the characteristicimpedance of a five wire transmission line is as follows: ##EQU1## whereZ=characteristic-impedance of five-wire transmission line

ε_(r) =relative dielectric constant for test medium

d=diameter of conductors

D=separation between outer conductors.

Thus the spacing and the wire diameters can be chosen to provide a tipwith matching impedance. For example, a test probe incorporated a 50 ohmcable 20 having an outer diameter of 0.141 inch, a shorting disc 50 ofthe same diameter and wires 0.036 inch diameter "d" having a separation"D" of 0.1 inch. For use with a 10.4 GHz source, the length of the tipshould be at least 0.8 inch. For clean oil (ε_(r) =2.2) the tip withthese dimensions has an impedance Z=55.26, according to the aboveequation. As the oil becomes contaminated the impedance match improves.For oil containing 5% water or soot, ε_(r) =2.5 and Z=51.84 ohms thusproviding a rather good match with a 50 ohm cable.

An advantage of the five wire tip is that the microwave field iscontained within the tip and is not radiated outwardly. Consequently,metal objects, that is the engine housing or other portions of theengine near the probe, do not influence the measurements made by theprobe.

While it is preferred that the tip have four outer wires, three or fiveor more may be acceptable as long as the probe meets the requirements ofimpedance matching, field containment and minimal oil hangup. As thenumber of wires decreases the field is contained less well, and as thenumber of wires increases the oil flow becomes more impeded.

Various well-known detection schemes can be used to determine thedielectric constant of the oil which in turn is interpreted as a measureof the degree of contamination of the oil. In any case, the envelope ofthe standing wave inside the coaxial cable 20 is dependent on thedielectric properties of the oil medium. As the dielectric constantincreases this envelope will change. The dielectric constant or thecontaminant content can be determined by measuring (1) the voltage levelof the standing wave at a fixed position along the coaxial cable 20, (2)the null location of the standing wave along the coaxial cable 20, or(3) the operating frequency of the source required to keep the null orthe standing wave at a fixed position along the coaxial cable 20. Toimplement the option listed first above, as shown in FIG. 1, the diodedetector 34 coupled to the coaxial cable will produce an outputproportional to the voltage level of the standing wave at that positionand the output is displayed by a deflection of the meter 36 which iscalibrated to provide the oil contamination information.

In the case of using a microwave probe as a liquid level detector, thesame detection arrangement can be utilized. When the oil level drops toa point allowing about 0.1 inch of air or vapor to enter the tip theeffective dielectric constant experiences a large change so that a largescale deflection occurs on the meter 36 whereas in the case of oilcontamination the meter deflection would be relatively smaller. Bylocating the line coupler between a null and maximum value of a standingwave (for clean oil) and calibrating the meter 36 accordingly,contamination of the oil will cause meter deflection in one direction(higher voltage) and low oil level will cause meter deflection in theopposite direction. Still another method of detecting dielectricconstant changes is the already-known technique of measuring impedancechanges using three diode detectors spaced 1/8 wavelength apart alongthe transmission line as set forth in "Three-Probe Method of ImpedanceMeasurement", W. J. Duffin, Wireless Engineer, December 1952, pp.317-320. Thus a probe and its associated microwave circuit are notlimited to separate uses of liquid level detection and contaminantmeasurement but rather they may both be combined in a single applicationwherein as long as the probe is fully immersed in the fluid increasingcontamination levels are evidenced by one type of signal output and ifthe oil level decreases below a critical value another type of outputwill occur.

A specific example of laboratory apparatus used for measuringcontaminants in oil with the subject probe by sensing the null locationof the standing wave in the transmission line is a line couplercomprising an HP-X810B slotted line serially coupled to an HP-423Adetector 34 and an HP-415E SWR meter 36 and energized by an HP-8690Bsweep oscillator, all from Hewlett-Packard Corporation of Palo Alto,Calif. and a DBG-480 isolator 30 from Systron Donner, MicrowaveDivision, Van Nuys, Calif.

Although the meter 36 is illustrated as the functional circuit output,other well-known outputs could be used, such as a signal level detectorcoupled with an indicator to indicate when a predetermined condition hasoccurred or the measured data may be stored in a memory device for lateranalysis.

It will thus be seen that as described herein, the microwave probe is aninexpensive, rugged and compact device which may be made very small foraccess into restricted places and can be configured to have a tip withimpedance matching the adjoining coaxial cable to maximize thesensitivity of the probe and to minimize the difficulty in analyzing thesignal reflected from the probe.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:
 1. A microwave probe formeasuring the dielectric constant of a liquid comprising:a coaxial cablehaving an outer conductive sheath and a concentrically located innerwire, and a cage-like tip attached to an end of the coaxial cableincluding a center wire connected to the said inner wire, a three tofive outer wires each connected at one end to the sheath and surroundingthe center wire, the outer wires being spaced sufficiently to facilitatefree flow of liquid through the tip, and means shorting the distal endsof the said center wire and outer wires; the wire geometry and number ofwires being so selected that the impedance of the tip, when immersed inthe liquid, substantially matches the impedance of the coaxial cable. 2.A microwave probe for measuring the dielectric constant of a liquidcomprising:a coaxial cable having an outer conductive sheath and aconcentrically located inner wire; and a cage-like tip attached to anend of the coaxial cable consisting of five parallel wires shorted atone end including a center wire connected to the said inner wire, fourouter wires each connected at one end to the sheath and surrounding thecenter wire, the outer wires being spaced sufficiently to facilitatefree flow of liquid through the tip, and a shorting conductor connectedto the distal ends of the said parallel wires; the wire geometry beingso selected that the impedance of the tip, when immersed in the liquid,substantially matches the impedance of the coaxial cable.
 3. A microwaveprobe for measuring the dielectric constant of a liquid comprising:acoaxial cable having an outer conductive sheath and a concentricallylocated inner wire, the inner wire axially extending a fixed distancebeyond the sheath; a round conductor element having a diameterapproximately the same as the sheath diameter connected to the end ofthe inner wire, and four outer wires parallel to the inner wireconnected between the conductor element and the sheath to form acage-like tip with sufficient space between the wires to allow freefluid flow through the tip, the wire geometry being so selected that theimpedance of the tip, when immersed in the liquid, substantially matchesthe impedance of the coaxial cable.